This invention relates to an apparatus and method for testing lubricating properties of fluids and wear resistance of materials
Fuel system components employed in some modern ground and aviation equipment rely on the fuel passing through them for lubrication of sliding contacts. Some of these components experience extreme temperature and pressure conditions in operating engines. One such component is the fuel injector used in diesel engines.
One way to examine the efficacy of fuel compositions as lubricants and the resistance of materials to the wear mechanisms experienced in fuel injectors would be to construct full scale working units and run them in test engines, examining them afterwards for wear. This approach is both costly and time consuming. It is of great utility in the transportation industry to predict the efficacy of fuel compositions in providing lubrication and the wear resistance of various materials of construction without having to construct and operate full scale equipment under conditions duplicating the operating conditions to which the equipment would be subject when in use.
To reduce time and cost, the properties of lubricating compositions are often tested using a testing jig. Typical equipment used for this purpose uses a mechanism to impart motion between two samples of material with the lubricant of interest interposed between the samples. The lubricating ability of the lubricant under study is determined either by studying the rate of wear of standard sample materials with different lubricants under the same conditions of load and motion, or to measure the amount of torque transmitted between a driving mechanism holding one sample and a driven mechanism holding the other sample when a given lubricant is interposed between the driving and driven samples. Both schemes have been employed to model components in operating equipment. The scheme utilizing the study of wear rates in sample materials has the added benefit that along with studying the lubricating ability of different lubricant compositions, the same equipment can be used to study the effect on wear of different combinations of materials or different surface finishes when they are examined with standard lubricating compositions.
Both schemes have been employed in bench top scale equipment and attempts have been made to correlate the results thus obtained with the results obtained from employing full scale equipment in an operating engine.
The wear rate in a component subjected to sliding contact is dependant upon many factors. Some of these factors are: materials of construction; surface finish; contact load; the relative rate of motion of the surfaces; and the nature of the lubricant and its film thickness. The environment in which a component is employed can also contribute to performance of a lubricant. Factors such as temperature and fluids present in addition to the lubricating composition employed must also be considered when constructing equipment that approximates conditions in operating engines. Numerous schemes for attempting to accomplish this end have appeared in the prior art.
U.S. Pat. No. 3,166,927 to Sonntag et. al. discloses an apparatus for measuring resistance to relative motion of two annular test surfaces having angular movement relative to one another. The apparatus disclosed is capable of imparting either rotary motion between two surfaces or oscillating motion (via an arm and eccentric) between the test surfaces. One of the two rods is a driving rod. One end of the driving rod is fastened to a motor, the other end is fitted with a mechanism for holding a test sample and a mechanism for contacting one surface of that test sample under controlled load against one surface of a second test sample. The end of the driving rod bearing the sample holder, the sample holder itself, and any sample contained in the sample holder resides in a lubricant sample well capable of containing a quantity of the lubricant composition under test.
The other rod of the pair is a driven rod. One end of the driven rod also resides in the sample well with the driving rod. The end of the driven rod within the lubricant sample well is also fitted with a mechanism by which a sample can be mounted on the end of the driven rod. The driven rod is supported so that when the driving rod sample is contacted against the sample fastened to the driven rod it is not displaced by the force of the contacting load. The end of the driven rod that is not within the sample well is fastened to a dynamometer and torque tube arrangement for measuring torque transmitted from the driving sample to the driven sample.
A scheme is disclosed wherein the sample lubricant and the testing surfaces are contained in a controlled temperature and atmosphere chamber for varying the testing conditions with respect to temperature and pressure. This arrangement is believed by the inventors to counteract the inaccuracies inherent in friction testing machines due to test sample axis misalignment and non-uniform wear of test sample surfaces during a testing procedure.
U.S. Pat. Nos. 3,302,447 and 4,228,674 to Mertwoy discloses an apparatus directed solely at testing the lubrication properties of hydraulic fluids under the conditions of temperature and pressure such fluids typically encounter in use U.S. Pat. No. 3,302,447 discloses the basic device, a set of chucks holding balls made of suitable material in rotatable contact, with provision for heating the hydraulic fluid under test, and applying pressure to the working faces of the balls used in the tests, subjecting the hydraulic fluid to loading conditions. U.S. Pat. No. 4,228,674 improves upon the former design, inverting the testing jig to counteract buoyancy of the testing samples in the hydraulic fluid under test, and providing for the ability to employ a fluid in the chuck driving mechanism having a different viscosity than the fluid under test, eliminating testing inaccuracies arising from fluids of different viscosity effecting the ability of the drive mechanism to rotate the chuck containing the test balls. A further improvement disclosed in the ""674 patent is the use of a spring and tensioning mechanism to preload the test balls, substituting for lead weights employed for this purpose in the device disclosed in the ""447 patent.
U.S. Pat. No. 3,913,337 to Lindeman discloses an apparatus for testing the lubricating properties of various fluids and the wear properties of various materials. In the disclosed apparatus, one of two motors, chosen for high speed operation or low speed operation, rotate a disk of test material between two stationary samples of test material. The stationary samples are in the form of a rod or block. One face of each stationary sample is held in contact with the faces of the disk by a caliper mechanism. Said caliper mechanism permits operator adjustment of the force applied to the faces of the disk of test material by the stationary samples. The test disk is arranged so that while rotating it passes through a bath containing the lubricating fluid under test. The entire mechanism except for the motors is contained in a sealed box, and provisions are made for heating and pressurizing the fluid under test. The disk is analyzed for wear at the end of an operator selected period during which the disk is rotated at an operator selected rotational rate while the stationary calipers impinge the stationary samples against the disk under an operator selected load. The apparatus provides for transducers which can measure the drag exerted against the rotational effort of the motor whereby the lubricating properties of the test fluid can be evaluated.
U.S. Pat. No. 4,253,326 to Munnich, et. al discloses an apparatus which may be employed to measure film thickness and moment of friction simultaneously. Additionally, it may be used to evaluate the wear properties of materials. The apparatus consists of two sample holders, the driving sample holder is mounted on a rotating shaft, the driven sample holder is mounted on a hydraulic device capable of linear motion along the axis of rotation of the rotating shaft. The sample holders are adapted to support samples of materials having test faces with said test faces in contact. The material employed in the test faces is employed either to test the properties of a lubricant or to test the wear properties of the materials which form the test faces. The apparatus utilizes strain gages to measure the torque imparted to the driven sample holder, and thus provides a method of calculating the frictional forces imparted between the driving and driven sample. A capacitance device is also provided to dynamically measure the thickness of the lubricant film residing between the two sample faces during the testing procedure. Lubricating properties can be calculated dynamically as a function of load, sample rotational rate, and film thickness. The test samples and sample holders are mounted in a sealed chamber permitting variation of lubricant temperature and pressure, and a design is disclosed wherein the loading force between the two sample faces may be varied as a function of time by oscillating the pressure of the hydraulic system that applies a loading force to the contacted samples.
U.S. Pat. No. 4,311,036 discloses an apparatus for testing the lubricating properties of liquids over a range of pressures and rubbing speeds which are varied between a test surface fully wetted in the test lubricant and a condition of dry friction between the test surfaces. The disclosed apparatus consists of a rotating cylinder of test material immersed in a sample of the lubricant under test. Two fixed specimens contact the surface of the test material cylinder normal to the cylinder axis of rotation. A load is placed on the fixed specimens. The disclosed apparatus incorporates strain gages to measure the resistance of the cylinder to rotation when loaded by the fixed specimens. The temperature of the test lubricant is measured and controlled, and in addition the lubricant film thickness is measured. Measurement of lubricant film thickness is achieved by measuring the electrical resistance between the counter specimens contacting the cylinder and the test material cylinder. Further features of the subject apparatus are screw adjusters interconnected by a flexible worm drive to permit both manual and electric motor adjustment of the force with which the counter specimens are impinged on the test material cylinder face.
U.S. Pat. No. 4,939,922 to Smalley et.al. discloses an apparatus for evaluating sliding friction between materials and the wear rate of materials such as those used in conventional 4 cycle internal combustion engine valve trains. The disclosed apparatus consists of a test jig that supports separate devices for applying and measuring the forces generated between machine elements when a reciprocating member runs in contact a rotating eccentric member. The face of the reciprocating member in contact with the eccentric and the eccentric itself are made of a material whose wear or friction properties are to be tested. The eccentric is rotated by a motor connected to the shaft upon which it is mounted. The shaft is equipped with a torque measuring and recording device as well as a vibration damping device. The reciprocating and eccentric test materials are contained within a sealed chamber. In use the sealed chamber is filled with the lubricant whose lubricating properties are under test. The chamber is equipped with a thermostat and a heating loop that passes the test lubricant through a heating device to maintain or raise the temperature of the lubricant under test in a preset fashion. Lubricating properties are determined by plotting energy consumed in rotating the test specimen as a function of lubricant temperature for a given load between the faces of the materials under test.
U.S. Pat. No. 5,281,535 to Wei, et.al. discloses an apparatus and method for examining the friction between two test surfaces exposed to a variety of gaseous and liquid fluids. The test surfaces of the disclosed apparatus are contained in a sealed chamber capable of supporting a partial vacuum and equipped with facilities to generate plasma when the test fluid is a gas (e.g. atomic oxygen plasma generated in an oxygen atmosphere). The testing apparatus disclosed comprises a horizontally oriented disk of test material affixed to a shaft passing through the bottom of the test chamber. The shaft is rotated by an electric motor. A second test surface (a xe2x80x9ccontacting pinxe2x80x9d) is supported on an arm above the rotating test surface. The xe2x80x9ccontacting pinxe2x80x9d has the form of a rod with a rounded end, said rounded end contacting the rotating test surface. The supporting arm of the second testing surface contains a screw device for varying the pressure of the contact between testing surfaces. The arm itself is affixed to a jig that is adapted to a path of reciprocating motion normal to the axis of rotation of the rotating testing surface. The reciprocating device is actuated by an electric motor, the shaft of which is passed through a seal in the side of the testing chamber. The frictional forces between the contacting pin and the disk of test material are measured by a strain gauge interposed in the mount supporting the xe2x80x9ccontacting pinxe2x80x9d. Additionally, the rotating sample may be examined at the termination of the test for wear patterns and degree of material worn away during the test. Additionally disclosed is a scheme for introducing more than one fluid into the chamber during a testing cycle.
U.S. Pat. No. 5,515,712 to Yunick discloses an apparatus and method for determining the power consumed by an internal combustion engine when it is rotated. The apparatus consists of an electric motor capable of rotating a gasoline engine at operating speeds, a torque meter affixed to the gasoline engine output shaft, and various devices affixed to the fluid pathways within the gasoline engine for measuring component displacement, rotational speed, air flow, and gas entrained in the lubricating fluid of the gasoline engine. The testing method disclosed consists of driving the engine with one set of components and lubricant installed, then changing out components or lubricant and rerunning the test, comparing the energy consumption required for the two configurations.
Some of the testing schemes disclosed in the prior art are adequate to test the relative properties of fluids intended primarily to provide lubrication to machine parts in sliding contact. Other apparatus and testing methods disclosed in the prior art are good at predicting the intrinsic wear properties sliding contact between two materials in unusual environments such as space. These prior art schemes are not adequate to accurately test the lubricating properties of fuel compositions or the antiwear properties of materials such as is required when performing lubricity or wear determination for materials and lubricants employed in fuel injection equipment. ASTM testing standard D6079 has been shown to correlate well the wear mechanisms which the sliding components in full scale fuel injectors experience during operation, and is incorporated by reference here within. ASTM testing standard D6079 specifies a high frequency reciprocating contact is required to simulate the wear mechanisms present in fuel injection equipment. ASTM testing standard D6079 further specifies that test specimens used in testing the lubricity of diesel fuel compositions be a 6.00 millimeter ball contacting a 10 millimeter diameter disk, with the area of contact under a load of 200 grams. The standard additionally requires that the test samples be moved relative to one another through a linear reciprocating stroke of one millimeter at a rate of 50 stroke cycles per second. The standard also requires that the point of contact between samples be completely immersed in the fluid the lubricating properties of which are being tested. It is known in the prior art that submerging test samples in the fluid under test entails additional problems which cause the test to deviate from correlating well with the wear experienced by full scale equipment under operating conditions. A practical testing jig must therefore address the problem of maintaining the point of contact under a covering of the lubricating fluid under test, without having it submerged in a bath so deep that viscosity and buoyancy effect the accuracy of the test. This problem is addressed in the ASTM testing standard D6079 by regulating the sample size placed in the apparatus and avoiding excessive heating of the sample. Other requirements of the standard are designed to eliminate sources of error in correlating the wear pattern generated on the test materials with wear experienced in full scale equipment under operating conditions.
Conventionally, testing of lubricity in sliding contact is accomplished by interspersing a lubricating fluid between a mobile and stationary work face and measuring the relative ability of various lubricating fluids to reduce the drag imparted to the mobile work face by the stationary work face in contact with it. Alternatively, a testing apparatus may utilize a driving and driven work face, and the effort required to resist motion in the driven work face is measured. In addition to measuring the amount of toque transmitted through a lubricant film, the type and amount of wear (generally measured as loss of material in one or both work faces) is observed on the workpieces after a period of sliding contact has been maintained between two work faces with the lubricant interposed between the work faces. The amount of material loss undergone by the samples with a standard lubricant interposed between sample faces will yields a method of evaluating the antiwear properties of the materials of construction employed in the test pieces. Standard materials and surface finishes can give a relative understanding of the lubricating properties of different lubricant compositions.
The above cited prior art apparatus may be divided into two broad categories: open chamber and sealed chamber equipment. The open chamber equipment is unsuitable for testing fuel compositions at the temperatures typically experienced by a fuel injector in an operating engine. Even if it were possible to keep the volatile fuel composition proximate to the friction surfaces of the test apparatus, the explosion hazard represented by open air testing of fuel compositions at or near their flash point would be unacceptable from a safety standpoint. Therefore, all apparatus which do not provide for closed chamber containment of the fluid undergoing lubricity testing represent a hazard when used to test diesel fuel compositions for lubricity at temperatures similar to those experienced in a fuel injector environment.
Closed chamber equipment described in the prior art may be divided into two broad categories, that which affords reciprocating contact between test surfaces and that which employs some other form of rubbing interaction between the surfaces. Since a reciprocating interaction is required to simulate wear conditions existing in a fuel injector, only a testing apparatus that provides for reciprocating contact would be adaptable to testing the wear resistance of materials and the lubricity of diesel fuel compositions in an apparatus which simulates the conditions experienced within a fuel injector apparatus installed in an operating engine. On this basis, only U.S. Pat. Nos. 3,166,927, and 5,281,535 relate equipment that provides for both reciprocating contact between test specimen surfaces and a construction incorporating a sealed chamber. All of the other prior art examples can not a priori meet the requirements for carrying out testing in accordance with ASTM standard D6079.
The apparatus disclosed in the ""927 patent provides for contact between planer specimens, precluding meeting the ASTM D6079 requirement to simulate fretting type wear by utilizing contact between a flat specimen surface and a spherical surface. Further, the design of the apparatus is such that there must be evenly distributed contact about the radius of the contacting surfaces, precluding introduction of a specimen containing a single off-axis spherical feature. Spherical contact could be achieved in the apparatus disclosed in the ""927 patent by machining spherical features evenly distributed about the radius of the one test surface. Using such a scheme however would require very precise machining of the specimen to insure that the spherical surfaces distributed about the radius were in precisely the same plane. This would be required because the design of the disclosed apparatus applies contacting pressure distributed evenly about the specimen""s radius. Any contacting surface protruding beyond the others would prevent the others from contacting the flat specimen until it had worn away an appropriate amount.
Secondly, the apparatus disclosed in the ""927 patent is directed at measuring lubrication properties by measuring the torque transmitted from a driving sample to a driven sample. ASTM standard D 6079 calls for a mobile sample surface to be displaced across a fixed sample surface. Since the moving sample surface disclosed in the ""927 patent (driving sample surface) is capable of displacing the stationary sample surface (driven sample surface), the disclosed apparatus is not capable of measuring wear properties in the manner specified by ASTM D 6079.
U.S. Pat. No. 5,281,535 discloses an apparatus which employs a worm drive carriage to impart reciprocal motion to a first sample surface relative to a second sample surface. The second sample surface is mounted on a spindle which can be rotated such that the reciprocal motion is perpendicular to the axis of rotation. The spindle and worm drive carriage are enclosed within a sealed chamber and drive by motors mounted externally and coupled with magnetic coupling devices. The arrangement permits reciprocal motion of one test surface against a second stationary test surface. The disclosed apparatus uses a magnetic coupling device to drive the worm gear assembly contained within the chamber. The reciprocal movement of the worm driven carriage is accomplished by reversing the driving motor. The apparatus disclosed in the ""535 patent is not capable of precise stroke control at the frequency required by ASTM standard D6079, and therefore is not suitable to carry out the test protocol specified by that standard.
One aspect of the present invention is that it affords making measurements of the lubricating properties of fuel compositions in keeping with ASTM standard D 6079. Another aspect of the present invention is that it affords lubricity property testing under the extremes of temperature and pressure to which machine elements are typically exposed when located in a combustion chamber environment, such as is the case for diesel engine fuel injectors.
Another aspect of the present invention is that the apparatus affords a method of testing the lubricating properties of fuel compositions near their flash point without danger of fire or explosion.
Another aspect of the present invention is to provide a method testing the wear properties of different materials and combinations of materials in reciprocating wear contact in the presence of fuel compositions under conditions of high temperature and high pressure. Another aspect of the present invention is to provide an apparatus which contains such safety features that the danger of fire and explosion inherent in testing fuel compositions at or near their flash point is minimized in the event of an accidental detonation or combustion of such a sample. Yet another aspect of the present invention is to provide an apparatus that minimizes the maintenance and impact of mechanical seals by providing a testing chamber that is sealed with a compressed fluid that is constantly renewed.
Other aspects of this invention will appear from the following description and appended claims, reference being made to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.